Asbestos is the commercial name attributed to several natural minerals having a fibrous structure and belonging to the class of silicates. In modern times some of these minerals were widely used because of their excellent technological properties: they have good resistance to heat and fire, to the action of chemical and biological agents and to abrasion and wear, display high mechanical strength and good flexibility, easily bind with construction materials and have good sound absorbing and heat insulating properties. Because of all these properties and its low cost asbestos was widely used in manufactured products and industrial and building applications, in means of transport and in the domestic sphere. In particular, the raw fibre was processed in order to obtain various products adaptable to multiple uses. In these products, the asbestos fibres may either be free, or strongly or weakly bound. If they are weakly bound, they are referred to as friable materials, which can be crumbled by hand pressure alone due to the poor internal cohesion. If they are strongly bound, they are referred to as compact materials, which can be crumbled into powder only with the aid of machinery. The materials in a friable matrix are undoubtedly the most dangerous, as the fibres can be dispersed into the air with extreme ease and thus inhaled. Asbestos in a compact matrix, given its nature, does not tend to release fibres and a hazardous situation may arise only if it is abraded, deteriorated or sawed. There are a vast number of types of Asbestos-Containing Materials (ACM) which have extremely varied and differentiated characteristics and uses. The U.S. Federal Register lists over 3000 finished objects which contain asbestos. ACM can be classified into three categories:                (a) Surface Materials: these include ACM sprayed or distributed by spreading over surfaces (weight-bearing elements, walls, ceilings) for soundproofing, heat insulating and decorative purposes;        (b) Heat Insulation Materials: these include the ACM used to prevent the formation of condensate in pipes, ducts, boilers, tanks and in various components of water cooling systems, as well as in heating, ventilation and air conditioning systems;        (c) Sundry Materials: this category embraces all of the other ACM, as in false-ceilings, sheaths, fabrics, etc.        
Asbestos has been undoubtedly most widely used in the building sector, in particular in the form of a composite of asbestos and cement, or so-called asbestos-cement. Moreover, in order to avoid or limit damage to constructions in the event of a fire, asbestos was largely used as a coating on beams or floors, applied with spraying and spreading techniques. The heat-resistant mixture was composed of varying percentages of asbestos and other materials (vermiculite, sand or cellulose fibres) and binding materials (gypsum and/or calcium carbonate): the result was a continuous layer, soft to the touch, of a colour varying from dark grey to white. Asbestos minerals were used as additives in cement conglomerates to improve their mechanical characteristics: the phases were usually Portland cement, water, aggregates and fibres of Chrysotile, Crocidolite and/or Amosite (more rarely), until eventually only Chrysotile was used. The asbestos content was variable and could reach 50% by weight depending on the type of product to be obtained.
Today it is a universally recognized the fact that asbestos is one of the materials most hazardous to human health among those present in living and work environments; this hazard results in severe pathologies prevalently affecting the respiratory tract. Although an etiological connection between the inhalation of airborne asbestos fibres and the onset of specific diseases was already hypothesized at the start of the last century, it was not until the 1990s that regulations consistent with the hazardousness of the material were introduced in various countries.
Ascertainment of the harm that this raw material caused to workers obliged the governments of all countries in the world to address the problem, in consideration of the exceedingly high social costs ensuing from occupational diseases developed by industry operators over the years.
It should be noted that the accumulation of Asbestos-Containing Waste (ACW) in landfills does not solve the problem, but rather simply passes it on to future generations: it is thus important to devise a strategy that allows ACW to be transformed and subsequently exploited as materials in the production of new products which are totally safe from an environmental viewpoint.
There are currently in use a number of processes, in addition to ACW “inertization” and “isolation”, which are suitable for transforming it and have the aim of completely eliminating the hazardousness thereof. “Inertization” processes include procedures for conditioning in matrices of varying nature which prevent the dispersal of asbestos fibres in the environment, whereas “transformation” processes act directly upon the fibrous structure of the mineral itself, transforming it into other phases that are not hazardous to human health.
The main ACW transformation processes are based on chemical treatments relying on the action of acids and thermal and mechanochemical treatments, though recently biochemical and microbiological methods have been devised.
Insofar as regards acid treatments, various methods have been developed which envisage the use of both organic and mineral acids to transform ACW so as to obtain secondary materials that are recyclable and often reusable in the ceramics industry. In particular, the effects of mineral acids, such as hydrofluoric, hydrochloric and sulphuric acid, as well as the effects of organic acids such as formic and oxalic acid, have been studied. As regards thermal treatments, it is well known that asbestos materials are unstable at high temperatures. Chrysotile, for example, has a tendency to lose the hydroxyl groups at around 600° C. and to be transformed into a different inert mineral phase, Forsterite, which is recrystallized at 820° C. The application of this principle makes it possible to obtain inert materials from ACW, as such or ground, treated in furnaces at a temperature of 800-950° C. Furthermore, if heating is preceded by compacting of the material, the consequent disorientation of the crystals allows the final product to be used as electrical insulation or refractory material. This process takes the name of ceramization. It is also possible to achieve a vitrification of ACW through a number of processes which are based on melting asbestos-containing waste with the addition of different additives within a broad temperature interval (1300-1800° C.), followed by rapid cooling with the production of an inert material having an amorphous vitreous structure. However, this solution requires a great deal of energy in order to bring the melting ovens to extremely high, constant temperatures.
In vitroceramization, on the other hand, the waste is melted at temperatures of between 1300 and 1400° C. together with particular additives, such as blast furnace slag or industrial sludge, forming a mixture with a high metal content. The slag thus derived is made to crystallize at a controlled temperature: in this manner one obtains products with very high mechanical strength, particularly suitable as coating and protective surfaces in the building, mechanical and chemical industries.
Another technique consists in so-called lithification, which is based on melting ACW derived from the removal of insulation from railway carriages at a temperature of 1300-1400° C. Slow cooling brings about a crystallization of pyroxenes, olivine and iron oxides. The final result of the treatment is the production of inert materials, which can be recovered for a variety of applications.
As regards biological treatments, the microbiological action of mosses and lichens on different rocky substrates containing asbestos fibres has been studied both in vivo and in vitro: the hyphae of lichens and fungi are capable of penetrating and secreting chemical compounds (oxalic acid is one of the primary metabolites), some of which can alter the mineralogical structure of asbestos fibres (see for example the article by S. E. Favero-Longo, M. Girlanda, R. Honegger, B. Fubini, R. Piervittori; Mycological Research, Vol. 111, Issue 4, pp. 473-481 (2007)).
Microbiological methods have also been developed for the transformation of asbestos using bacteria, in particular Lactobacillus casei and Lactobacillus plantarum (see for example the article by I. A. Stanik, K. Cedzyńska, S. Żakowska; Fresenius Environmental Bulletin, Vol. 15, Issue 7, pp. 640-643 (2006)). The method is based on breaking down the crystalline layers of Brucite (magnesium-oxygen) present within the crystalline layers of Chrysotile as a consequence of the indirect metabolism of the bacterial cultures used. The decomposition of crystalline layers seems to be due to the acidification of the reaction environment, thanks to the presence of metabolites secreted by the bacteria, which also include lactic acid. The hypothesized reaction mechanism is achieved through a substitution of Mg2+ ions by H+ ions, which are present in great excess. The magnesium thus released reacts with the lactic acid present to form soluble salts.
One of the microbiological processes for decomposing asbestos fibres (mainly Chrysotile) contained within asbestos-cement products was patented by Chemical Center S.r.l. (European patent: EP2428254), a company operating in the sector of analysis and in particular eco-innovation.
The process envisages using amounts of exhausted milk whey having an acidic pH to break down the cementitious phase (85%) and release the asbestos fibres (15%) incorporated therein, fibres which are then denatured and broken down into magnesium ions and silicate using further amounts of exhausted milk whey in a hydrothermal process. The overall process can be divided into two steps: 1) decomposing the calcium carbonate so as to release the asbestos fibres in water and 2) decomposing the asbestos fibres.
Unfortunately, the methods for transforming asbestos-containing materials (ACM) known to date present non-negligible disadvantages. In particular, acid treatments lead to the accumulation of a large amount of waste products, which also need to be disposed of. Furthermore, it should be kept in mind that in order to treat millions of tons of ACW (the approximate estimate for Italian territory alone ranges between 20 and 30 million tons) it would be necessary to use enormous amounts of reagents, which would entail non-negligible environmental risks and very high costs. With regard to thermal treatments, the largest disadvantage, besides the enormous amount of energy required to bring the furnaces to very high, constant temperatures, is given by the fact that suitable equipment is often polluting and highly costly and thus scarcely available across the territory, so that it is necessary to transport the ACW over long distances, with the consequent environmental risks and logistical costs.
The processes that use biochemical and microbiological methods (including the process that envisages using milk whey) also present several disadvantages, such as, for example, a low degree of transformation of the asbestos fibres, sometimes occurring only superficially without reaching a complete transformation. Therefore, such methods have not found to date any applications that are feasible on an industrial scale.
In particular, the use of bacterial microflora of Lactobacillus (envisaged in the method of EP2428254 and in the article of I. A. Stanik et al. Fresenius Environmental Bulletin, Vol. 15, Issue 7, pp. 640-643 (2006)), requires a culture temperature of 37° C. in order to obtain acidic metabolites, particularly lactic acid, and reach an acidic pH that is efficient in decarbonising the calcite phase of the asbestos-containing material. Moreover, the culture times for obtaining a microbial population that is sufficient for denaturing are generally long.
In addition, a characteristic of milk whey that is disadvantageous for denaturing an asbestos-containing material is the lipid component, which, by forming micelles of fat at the water-air interface, causes a slowdown in the carbon dioxide decarboxylation reaction and thus an equilibrium toward the re-precipitation of calcite.
Moreover, the excessive biological component, both lipidic and proteic, interacts with the asbestos fibres, enveloping them with a protective biofilm, packing them together and making the denaturing thereof through an ionic exchange reaction more difficult.
Ultimately, milk whey is mostly used for zootechnical nutrition and only in certain periods of the year is there a certain availability in the market as actual waste from dairy production.